


Fanning The Flames

by MeGaLoTrash



Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell, Breeding, Crying, Ectobiology, Fluff and Smut, Knotting, Love Confessions, M/M, Masturbation, Mating, Mating Cycles/In Heat, Minor Violence, Mutual Pining, Olfactophilia, Oral Sex, Overstimulation, Possessive Behavior, Rough Sex, Underfell Papyrus, Underfell Sans, Unresolved Sexual Tension
Language: English
Status: Completed
Published: 2018-05-02
Updated: 2018-05-02
Packaged: 2019-05-01 08:57:04
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 6,957
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14516928
Author URL: https://archiveofourown.org/users/MeGaLoTrash/pseuds/MeGaLoTrash
Summary: For @Bigbitchbot on tumblr who won 1st prize in my raffle, they asked for an intense fellcest heat fic, and i hope i've managed to pull it off ;)Extra warning re: assault in the end notes for spoilers.





	Fanning The Flames

“fuckin’ stubborn ass bitch.” Sans grumbled as soon as he closed his door, kicking his night stand in frustration before flopping onto his bed.

Sans was done. He had just endured the most humiliating dressing down in public from his brother, his face was still burning in shame hours afterwards. Granted it was entirely his fault for spending the better half of his shifts all week at Grillby’s, but he needed to drown out certain things more so than usual just to stay sane. Sans had been harbouring a deep but secret attraction to his brother for years. He knew it was wrong on so many levels, but Papyrus was perfect in his eyes and the fact that he couldn’t have him just made it so much worse. There was a time where he had thought his attraction was more than lust, but that had been stamped out by drinking Grillby’s top shelf and reminding himself of his brother’s mean streak. But nothing he did could dissipate images of Papyrus above him every time he jerked off. 

For some ungodly reason, Sans’s attraction had decided to double if not triple in intensity in the past week. His fantasies were no longer confined to when he was fucking his hand either, lewd images of his brother were somehow filtering into his thoughts at every opportunity. Every touch, stolen glance, hell even being within ten feet of Papyrus was driving Sans insane. Which was why he had been avoiding him as much as possible, seeking out the comforts of Grillby’s to try and destroy those feelings or die trying. He saw the looks of concern from the bartender and some of the regulars, but he kept on going regardless. Everyone would agree he was doing the right thing if they knew the truth of his sick desires. 

But a week went by and either someone had ratted him out, or Papyrus had deviated from his normal patrol and found Sans’s empty sentry station. Sans didn’t care, the outcome was the same. His attempts at an explanation were weak even for him, but he was so tired of everything that he had already accepted his fate. Papyrus didn’t get physical with him, his tongue was sharp enough that there wasn’t any need to demonstrate his strength. But Sans had wanted him too. He had zoned out of Papyrus’s lecture thinking about his brother hurting him in a different way, or of Sans being the one to bring Papyrus to his knees, or of them both clawing at each other as they fucked mercilessly-

_not this shit again!_

Red groaned into his pillow. Even after what Papyrus had just done he still couldn’t get the thoughts of screwing him senseless out of his mind. But his brother was being a fucking asshole! They had an unspoken rule that they didn’t bring work home, that their house was meant to be a respite from all of the shit they had to put up with in the underground. Sans might have been distant recently, but Papyrus was being unreasonable. While Papyrus didn’t lecture him more, he was colder than Sans had previously thought possible. Every look he gave Sans was filled with malice. Sans felt like he had to walk on eggshells all night to avoid whatever rage was bubbling just below the surface on his brother’s face. He had tried to talk to Papyrus on the way to bed but instead his face had become well acquainted with his brother’s bedroom door. 

Red was infuriated but for some fucking reason his magic had decided that the appropriate response was to pop a boner as soon as he reached his room. 

Sans thought he would be over this by now, Papyrus should have done a good job at putting a damper on things for at least one night. But no, their confrontation had only fanned the flames. It didn’t help that when Papyrus had been yelling at him he had got close enough that Sans had been able to fucking smell him. Sans knew what his brother’s scent better than his own, but what had previously been mundane suddenly felt became irresistible. He cringed thinking about how he had inhaled deeply in front of the whole bar. But considering the circumstances everyone probably thought he was crying or something equally as pathetic. It was embarrassing, but anything was better than the truth. 

A few lazy strokes through his shorts was all Sans could bare before he had to flip himself over and arrange his lumpy bedding into something he could fuck. He didn’t usually put in this much effort unless he had a rare night alone in the house, but Sans felt so pent up that he knew his hand wouldn’t be enough. His hips needed to fuck the aggression out with each thrust and he needed to dig his hands into something until they hurt. Since his brother wasn’t an option, his bedding was going to have to do. They were already ruined by this hell of a week, he might as well fuck them up more. Sans passed out with a wet crotch and a fuzzy head. He resigned himself to a restless night dreaming of sinking into a warm, wet heat that just happened to be attached to his brother.

Morning came far too soon. Sans had to double check the time to realise he was actually awake earlier than he had been in months. He didn’t have to wonder why for too long though, he was pitching quite an impressive tent in his ruined shorts. 

“oh come on!” Sans grunted, covering his head with his arm as he willed his magic to go away. 

It was unbelievable; Sans was usually a one shot and you’re done kinda guy. He couldn’t have gone to bed less than 6 hours ago and yet his dick felt like it hadn’t been touched in weeks. It made no sense, but it wasn’t like he had the time to deal with it either. Papyrus was already up if the clattering in the kitchen was anything to go by and if Sans wanted to get onto his good side then he needed to drag his ass out of bed. But first, he needed to get out of his ruined clothes that were sticking to him from a mix of sweat and cum. He thought about grabbing a shower but hesitated. If he could put up with smelling like he’d run the entire length of the underground then hopefully he’d minimise the risk of picking up his brother’s scent again. Sans grabbed some clean-ish clothes, flipped the cock that refused to disappear up against his waistband, and hoped his sweater was thick enough to hide the glow. 

As soon as he stepped out of his room he was assaulted by the most incredible smell coming from the kitchen. His soul actually swelled at the thought of his brother wanting to make it up to him by making breakfast. It smelled more than edible and Sans found himself taking the stairs two at a time just so he could get to the kitchen faster. But when he turned the corner and found nothing but his brother in the kitchen he was confused. Papyrus looked like he was in pain, his legs shaking while he leaned against the countertop. Sans almost rushed to his side when another wave of that intoxicating smell hit him like a ton of bricks. 

Usually Sans would curse himself for thinking with his dick, but the painful throbbing against the seam of his pants made him freeze. He had to smack a hand over his mouth as everything slotted into place. The skyrocket in his libido, the intrusive thoughts, the smell coming from Papyrus, and the way he looked like he was subconsciously rubbing his legs together. 

Papyrus was in heat. 

But this made no sense!! No one went into heat here, not unless they had a fucking death wish. Heats were a pipe dream from a long-gone era of when monsters felt safe. But with monsters falling down every week from an underground ruled over by a psychotic tyrant, no one had willingly conceived in years. Even the married dogs weren’t stupid enough to let themselves become so vulnerable unless they had the king’s protection. Bonded couples were so rare that the king himself took interest in making sure they had the opportunity to mate. For the prosperity of the kingdom or some other bullshit. It was one of the reasons that they were so valuable to Papyrus’s unit, monsters would think twice about touching those with the king’s favour. 

What was worse though was Sans could feel his whole body reacting, he could feel the overwhelming urge to rush to his side in his bones. It seemed odd that Papyrus’s heat wasn’t making him want to roll over and beg his brother to fuck him stupid. Sans was struggling to stay still though, his body practically willing him towards the soft swell of magic he could see in his brother’s pants. It seemed that his brother would have more than a cock summoned, but with how tight his pants were it became obvious that it wasn’t a dick in his pants. The second realisation hit him harder than the first, actually making him let out a strained noise. Papyrus wasn’t just in heat, but he was in a submissive heat. This was usually reserved for weaker monsters, ones that had to rely on others. Exactly the opposite of his brother. Unbelievable as it was it did make a lot of sense considering Sans hadn’t been able to get fucking Papyrus out of his-

“Have You Quite Finished Gawking At Me Welp?” Papyrus snapped Sans out of his thoughts by turning to face him, though his voice lacked any of the malice it had last night. 

Sans took a moment to look at his brother in an entirely new light. He was still able to function despite the now obvious heat, the glimpse of weakness Sans had seen when he had entered the room was all but gone. But everything about him seemed softer somehow. He may not have had a full body formed but the tightness of his pants outlined every curve and swell around his crotch. Papyrus still looked pissed off, but he didn’t look like he was debating dusting Sans anymore. Even his voice had been softer, the quieter tone like a siren trying to draw Sans closer so he could hear him better. Sans had somehow managed to walk within a foot of his brother without meaning too. The only Reason he snapped out of his trance was from Papyrus coughing loudly to remind him that he _still_ hadn’t said a word. 

“shit- yeah boss- i was just.” Sans trailed off when he noticed the faint dusting of magic on his brother’s cheeks. He wasn’t sure if it was from embarrassment or the heat, but he didn’t care. Sans had never seen anything more endearing. 

_but that’s fucking wrong_.

Sans shook himself in disgust. He was so hypnotised by the effects of the heat from the only monster he’d truly cared for that he had forgotten all sense of reason. This was his _brother_. Even if Papyrus knew what was happening to him, which he doubted considering how stubborn he was about his ‘weaknesses’, it didn’t change the fact that Sans would never cross that line. _unless papyrus wanted too of cours- no!_. Sans shook himself out of that line of thought. It didn’t matter, but he couldn’t let his brother go out in this state. Sans might be a creep, but he’d never force himself on Papyrus. He couldn’t say the same about everyone else. Sans needed to think of an excuse to keep him safe and at home until he figured out what to do. No matter how hard it was going to be for Sans around his brother, nothing was worth putting him at risk. 

“ya look a bit pale pap.” Sans laughed nervously when Papyrus raised his brow bone, knowing full well that there was nothing paler than their ivory bones. “well not pale but off- like summits wrong. sure ya don’t wanna call in sick today, lay down fer a bit?”

“Sick?! The Terrible Papyrus Doesn’t Get Sick!” 

Sans could have screamed. Papyrus was already crossing his arms defensively and walking away, clearly his mind was made up despite the fact it was obvious he wasn’t in the right. Even if he was in denial about the heat, there was no denying he was in a weakened state. Sans followed him across the room, mentally preparing himself to be rejected again. 

“pap, there’s no need to pretend.” Sans tried to reassure his brother by grabbing his arm, pushing down the way his fingertips burned from the lightest of touches. “it’s just me.”

“I Am _Not_ Sick.” Papyrus spat back before shrugging him off and moving his hands to his hips, being stubbornly defiant despite Sans having boxed him into a corner of the kitchen. “And Stop Calling Me That- That Childish Nickname! I’m Your Superior And Deserve To Be Addressed Properly.”

Sans sighed heavily, making things worse for himself as it made him inhale nothing but Papyrus’s scent. He knew his brother was only doing this because he felt out of control, but it was just pushing all of his buttons. 

“we’re at home.” Sans replied through gritted teeth. 

“So?” Papyrus smirked when Sans exhaled heavily again, clearly enjoying rattling his brother’s bones. “I’m Sick Of That Stupid Rule, I Give You Far Too Much Leeway And You Slack Off All The-

“stop.” Sans slammed his hands on the wall behind Papyrus hard enough for him to yelp. He knew he had gone too far, and that being this close was dangerous, but he was fucking livid. “ya know god damn well why we have that rule papyrus.”

The air hung heavily between them as neither of them dared to even breathe. For a moment Sans was sure he was going to answer back, but the heat reared its head instead. For a second it felt like they were in a very different situation. Papyrus looked at him with lidded sockets as his spine arched off the wall and pushed his pelvis centimetres away from Sans’s crotch. It was clear the reaction wasn’t voluntary, with Papyrus shaking in shock as Sans hissed through his teeth. Neither of them had done a good enough job at hiding their reactions, shifting the atmosphere between them from tense to unbelievably awkward. 

“Yo-You’re R-Right, S-Sorry” was the only thing Papyrus muttered as Sans slowly backed away. 

Sans didn’t dare meet his brother’s eye as he darted past him, completely stunned that he’d let it get that far when he _knew_ how fucked this all was. He could hear that Papyrus was gathering his things slower than usual, but those noises felt distant compared to the lecture he was giving himself. Sans had been so close to snapping and taking what he wanted from his younger brother. Hell, he was still teetering on the edge of turning around and slamming him into the wall again now. Just thinking about how he’d managed to wipe that fucking smug smirk of Papyrus’s face was really doing it for him, he could feel his cock starting to leak in his shorts. 

But through sheer force of will and a self-deprecating inner monologue, Sans managed to keep himself in check long enough for his magic to stop throbbing. Papyrus was ready to leave by the sounds of it, but Sans still took care in turning around just in case the sight of his brother made him lose it again. He was right to be cautious. Papyrus for some unknown reason had thought it was a good idea to be stood at the front door _wearing his fucking coat_. Sans was hit with a range of emotions from outrage to desire, making his mouth fall open in shock. But what was rapidly overpowering everything else was a strong feeling of possessiveness overseeing Papyrus in something of his. Sans didn’t want him to ask him to take it off, he really wanted everyone to think that Papyrus was his. 

“Br-Brother I Have Decided That If I Were To Get Sick That Would Not Be Acceptable SO I’m Taking Your Coat T-To Keep Warm.” Papyrus had gained some of his confidence back but he was still backing away, his face still a pretty shade of crimson which was making Sans’s soul ache. “Which Means You’ll Have To Stay Home, Here. And Not At Your Post In Snowdin Where I Will Be Patrolling.” He added, practically tripping over himself to get out the door and giving Sans no chance to respond bar opening and closing his jaw like a fish out of water. “Ok, That’s Settled SeeYouLaterBye!”

Sans felt the vibrations of the door slamming when he screwed his sockets shut, allowing himself to groan loudly now that Papyrus was intent on running away. He might have been letting his usual restraint around his brother slip, but Papyrus was being just as stupid if not more so. Sans was no longer so sure that Papyrus was oblivious to what was happening. At least on some level he was aware that something was happening to him, and that it was the cause of what had just happened in the kitchen. At first it stung that his first reaction was to run away, but then Sans couldn’t be too hard on him. Papyrus’s body might have been responding to him, but that didn’t mean his mind was going to just fall in line and want to fuck him.

No matter what, heat or not, nothing changed the fact that they were brothers. And Papyrus wouldn’t want to fuck his brother if he was in his right mind. No matter how much he had looked like he wanted Sans to overpower him and-

 _fuck_. Sans was practically creaming himself in his shorts just remembering his brother’s expression. There was no way he was going to be able to keep his sanity if he didn’t take care of himself before Papyrus returned. He was ready to lock the door and pull his cock out, but with Papyrus’s smell fading questions that he had over everything were starting to pop up. Monsters didn’t just go into heat, something must have happened for Papyrus to feel safe enough to go into heat. _but what?_. Sans tried to think of something that had changed recently but he was coming up empty. Which meant that it was most likely something he didn’t know about, the realisation making his soul sink. He’d been avoiding thinking about it at all, but the likelihood of Papyrus being with someone else felt like the obvious answer. 

The thought of another monster claiming his brother made Sans feel sick. Consensual relationship or not, he didn’t want to think about anyone having what he craved so badly. But he couldn’t believe that his brother would keep something like this secret from him, he was terrible at keeping secrets and often fake-bragged about how he was an expert in dating. So that left Sans with two likely options; either his brother was keeping some love interest from him because he’d figured out Sans’s feelings and was trying to protect him, or there wasn’t anyone else and Papyrus’s heat was because of _him_. 

Unfortunately Sans didn’t have the capacity to think more on the subject. Papyrus might have been far enough away for him to not be foaming at the mouth, but he was still a creep who couldn’t get the way his younger brother had looked pinned against the wall out of his head. Sans flung himself onto the couch and pulled his magic out of his shorts. Papyrus had all but chained him to the house, and while he was worried he knew that following him would do more harm that it would do good. He had to hope that furiously jacking off would minimise the risk of doing something he would really regret when Papyrus eventually came home.

_if he came home at all._

The rest of the day was absolute hell. Sans alternated between trying to sleep to pass the time and then having to take care of himself after waking up from fever dreams. He had lost count over the number of times he’d tried to jerk off, but after cumming once his hand just wasn’t enough. All he succeeded in doing was making himself more frustrated as his entire body cried out for something he couldn’t have. His mind was waging a war between panic and guilt; he wanted to go out and find his brother, but couldn’t trust himself enough to be able to hold back around him either. 

But the hours ticked by and Sans’s worrying eventually won. He had been rooting around his pile of clothing for something inconspicuous to wear when he heard the front door being flung open then loudly slammed shut. Sans ran out of his room fearing the worst but nothing could have prepared him for the way his soul plummeted upon seeing his brother trembling on the floor. The closer he got the more he could see that his coat was flecked with blood from multiple sources and that liquid magic was streaming from his brother’s eye sockets. Someone had hurt Papyrus, and Sans was going to dust the disgusting fucker who dared to lay a finger on his brother. 

“papyrus!” Sans pulled his brother’s face up towards him as soon as he reached him, instinctively wrapping a protective arm over whatever part of Papyrus he could reach. “what happened? are ya hurt? who the fuck did this?i swear i’m gonna-”

“I’m Ok Sans, It's Not As Bad As It Looks. My Health Hasn’t Been Impacted.” Papyrus replied, adding the part about his heath when he met Sans’s panicked gaze. “I’m Not Sure Who, I-I Wasn’t Thinking Straight And I Went To Look For You At Grillby’s-” He averted his eyes again as if that confession was embarrassing to him. 

Sans would have found that little reaction endearing if he wasn’t seething with anger. He brought Papyrus closer when his brother started to shake again, hoping that having him in his arms would help calm him down. A distant voice echoed in his skull warning him about being so close to Papyrus considering what had happened earlier, but he pushed it down. Sans was willing to put everything at risk to comfort him, even if it meant his brother would find out about his attraction to him. 

“There Were So Many Monsters There-” Papyrus shook and tightened his grip on the arms around him. He may have been the bigger of the two brother’s height wise, but he seemed so fragile in Sans’s embrace. It was soul-breaking. “They Kept Saying Weird Things, Like How I Smelled Nice And What They Wanted To-” Papyrus cut himself off with a sob that sounded more like a wail before burying his skull into Sans’s jumper. “-Brother I’ve Been So Stupid!”

The tears came fully then, with Sans torn on how to comfort him properly. He couldn’t remember the last time they held each other like this, but the comfort of the embrace was tainted with Sans’s anger. On the one hand, he wanted to sprint to Grillby’s and burn the place to the ground. But he knew that the repercussions would hurt them more in the long run. If he was being honest, he could rationalize that monsters weaker than them couldn’t resist Papyrus’s heat. He knew first hand how debilitating the effects could be, add ignorance and alcohol into the mix and it was a recipe for disaster. Sans cursed himself for being foolish enough to think that Papyrus would have been ok without his protection, he should have followed him earlier. Instead he let his own discomfort cloud his judgement, he shuddered to think about how much worse it could have been. 

“hey, ya not stupid. if anythin’ i’m the one who’s an idiot. ya know what’s-” Sans clenched his teeth tight when he noticed his voice sounded more like he was growling instead of talking. He had hoped that Papyrus’s heat would have dulled at least somewhat considering the shock he had been through but that wasn’t the case at all. Sans was suddenly very aware of the new rips on his brother’s uniform, the heat of his body pressing into him, and the sobs that sounded more like sensual whimpering. He could feel his mind becoming fuzzy as he struggled to resist his instincts to take Papyrus’s worries away by claiming him instead. It was so fu-

“S-Sans?” Papyrus had adjusted himself in Sans’s arms to face him, his eyes looking like they were almost sparkling with the tears that were yet to fall. 

It took Sans’s breath away, and he considered throwing everything away if it meant he could find out what it felt like to kiss his brother. Sans brushed his thumb across Papyrus’s face, feeling his chest rumble when his brother’s sockets became lidded again. _was it possible that pap wanted this, like really wanted it?_. Sans knew he couldn’t let him go now, his whole body was screaming at him to hold tight and never let Papyrus go. He scrambled to find reasons to resist him but his sanity was rapidly evaporating. He needed to talk to Papyrus; maybe hearing his brother actually rejecting him would be enough to douse the fire burning inside him. 

“i can’t think clearly right now so i’m gonna have t’be blunt pap.” Sans said through gritted teeth, hoping that he was doing the right thing and not fucking things up forever. “ya know yer in heat right?”

Just like he expected Papyrus froze. Sans was so busy berating himself for crossing the line that he almost missed the reply. 

“...Yes.” 

“so- why have ya come to-” Sans cut himself off, forcing his voice to a whisper even though he wanted to scream a million questions at the skeleton in his arms. Papyrus was still trembling as he averted his eyes, the rapidly growing blush on his face being the only give away that he had really just admitted knowing what was going on. “why the hell was ya looking fer me at grillby’s if you knew?”

“I-”Papyrus’s eyes flicked back to Sans’s in panic before his heat made him whine as more of that delicious scent radiated off him. He leaned into Sans, making their foreheads touch as he took in a few shaky breaths. “I Think We Both Know That There’s More Between Us Than We Want To Admit.”

It was Sans’s turn to freeze. All of the fantasies he’d had today of him and Papyrus being together had always been rationalized as a result of the heat only. But what his brother was saying rang clear. This couldn’t just be the heat, and there was no malice or mocking in what Sans could make out in his brother's eyes. This had to be real. Papyrus gave him a few moments to respond, but then fear flashed onto his face. It was a look Sans knew all too well, he just never thought he’d see it on his brother’s face. Especially not over his feelings for him, which until a few minutes ago Sans had thought didn’t exist. 

“I Know It’s Not Right But-” Papyrus cupped his face and Sans caught a sliver of his tongue darting out between his teeth. “I-I Can’t Stop Thinking About Being With You Brother.”

“shit pap, you don’t know what you’re asking.” Sans let out a breath he didn’t know he had been holding as his hands grasped at any part of Papyrus he could get hold of. He was close enough to feel his brother’s breath on his face and it was driving him crazy. “its gonna change everything. we can’t-”

“So? Avoiding Each Other An Arguing Has Made Me So Miserable”

“-but the guard. the king!”Sans shuddered as more of their bodies melded together. He may have been trying to talk sense into them both with his words, but his body had a mind of its own. He almost gave up when Papyrus moved into his lap properly, those long limbs wrapping around him as if they belonged there. “if they knew pap, you’d be taken from me, no one would respect you because of me and-”

“I Won’t Let That Happen. We Can Keep Up The Act To Everyone E-Else-” Papyrus stuttered when he sat down on his brother properly, the touch of their magic through their clothes feeling sweet as sin.“But I Have No Shame I-In Wanting To Be Truly Yours-Umf!”

Sans Snapped. Hearing that Papyrus actually wanted him was too much. He flipped their positions and pinned his brother to the floor, grinding their throbbing magic together while leaning up to kiss him. The heat did have some effect though, Papyrus practically squealed from the contact and moaned when Sans’s tongue tangled with his own. Papyrus might have been happy to submit to him but he wasn’t being passive at all, he re-wrapped his legs around Sans and shifted his pelvis up to meet Sans’s harsh grinding. It felt better than Sans could have ever imagined, all those years of frustration were melting away in the taste of his brother’s tongue.

But soon it wasn’t enough, the building heat between them requiring much less clothing than what they currently had on. 

Sans wasn’t worried about stripping himself, all he needed to do was pull his cock out when he was ready. The restriction was frustrating but it was helping savour the moment of each barrier breaking between them. But he felt the irresistible urge to see everything of the skeleton pinned underneath him. With his tongue still exploring Papyrus’s mouth Sans started to undo his brother's belt, growling in frustration when he couldn’t pull the tight pants off him. Papyrus whimpered when he pulled back but shimmied his pants down his legs as Sans watched with rapidly disappearing eye lights.

Sans knew Papyrus’s magic colour from watching him fight, but the scarlet ecto-flesh was incredible to see up close. Papyrus had formed magic from the bottom of his ribcage to his knees with the brightest glow coming from the plump pussy between his legs. Rather than be bashful about it Papyrus laid back down with a smirk on his face, obviously enjoying the effect his naked form was having on his brother. But Sans wasn’t one to be outdone. He thought about pulling his coat and Papyrus’s top off but decided instead to shred the top with his fingers and leave the coat on. He hadn’t been able to get the image of Papyrus in his coat out of his head all day, so he wasn’t going to pass up the opportunity to fuck him lying on it.

The sight was mouth watering. Papyrus was unable to keep still, writhing around on the floor as he let go of his restraint and gave into his heat. His formed body was covered with a sheen of perspiration, but Sans was drawn to the dripping magic between his legs. Sans leaned into his brother for another kiss, pressing their bodies flush together and rubbing his clothed cock against Papyrus’s pussy. He could feel Papyrus’s magic soaking his shorts while he deepened the kiss making him shiver with anticipation. He wasn’t able to stop his hands from trailing down his brother’s ribs, groaning in appreciation when Papyrus arched into his touch. 

Papyrus whimpered when Sans shifted off his crotch, but was quickly subdued when Sans’s hand moved lower on his body. Sans was staring into his brother’s eyes when his fingers slid across the lips of his pussy, soaking up every last detail of how his face contorted in bliss. Papyrus shoved a hand over his mouth to try and stop his moaning but he wasn’t quick enough, the melodic tone of his cries echoing around Sans’s skull. Sans was sure he was flushed himself over actually doing this to his brother, but he still moved Papyrus’s hand away. He was meant to be taking charge here, even if he felt like he was going to turn to jelly from just watching Papyrus writhe underneath him. 

“Boss, yer soaking” Sans whispered, hoping he was reading his brother right and that a little teasing was exactly what he needed. He moved his fingers slowly across his brother’s cunt, mapping out all the points on his magic that made him squirm. At the moment it seemed like everything was driving him crazy, which made Sans pause. “this isn’t just the heat is it, you really want me?” he asked, slowly circling around Papyrus’s entrance to try and coax the answer out of him. 

It started to work, with Papyrus gripping hold of his arm and forgetting all about keeping himself quiet. “don’t make me say it- ah!”

“how long?” Sans continued his teasing by dipping a fingertip inside just to draw it back out. He could feel his own magic pulsing from the way Papyrus was humping his hand but there was no way he was going to rush this. “pap you don’t know how bad i’ve wanted you for fucking years. i’ve dreamed about doing this-” he moved his fingers back to circle his brother’s engorged clit to drive his point home “-and about how fucking sweet it was gonna be to taste ya” he gave Papyrus a wink before starting to move down his body, his implications crystal clear. He paused when he was level with his brother’s pussy, the incredible smell of his heat making it hard to keep teasing him. “but i wanna know. how long have you wanted me?”

“Since- Since- Hng-” Papyrus shivered when he felt Sans’s breath ghosting over the heat of his magic, the chills making it hard to focus. “Since Joining The Guard.”

“you mean making captain?”

Papyrus’s face coloured even further before he confessed the true extent of what he had been trying to hide. “...No.” 

Sans wouldn't believe what he was hearing. Papyrus had been with the guard since he was barely out of striped shirts. “fuck boss, i had no idea” Tears were welling up in his own sockets as the years of missed opportunities flashed before him. But there was no point in getting worked up about that, not while Papyrus was so willing and spread out for him _now_. “we best start making up fer lost time.”

Sans didn’t waste any more time teasing. He sunk two fingers into his brother’s heat and gave his clit a slow lick with a flattened tongue. Papyrus was so overwhelmed that he jerked and shifted to his side out of reflex, bringing Sans along with him. Sans didn’t mind too much, he moved one of Papyrus’s legs over his head and held onto it with his spare hand while continuing to eat his brother out. The heat heightened Papyrus’s taste, but Sans knew that he would never get tired of anything his brother had to give. With their new revelations he knew that he was going to spoil Papyrus with attention, but for right now he had to work at satisfying his heat. 

Sans started gentle but it became clear that Papyrus responded well to rougher treatment. It felt like they were a match made in heaven, with Papyrus practically screaming every time Sans followed his instincts to keep doing more. Sans paid attention to his brother’s breathing and the way his legs were twitching, knowing that he couldn’t be far off his first climax. He shifted his head upwards to suck on his brother’s throbbing clit while hooking his fingers upwards. The result was instant, Papyrus was brought to a screaming climax with Sans riding him through while his soul soared in ecstasy. 

“better?” Sans asked when he eventually drew back, intending to crawl back up his brother’s body and share another heated kiss. 

He didn’t expect Papyrus’s legs to wrap around him and drag him closer until the head of his neglected cock was pressing at his brother’s widened entrance through his shorts. 

“N-No! I’m Burning Up” Papyrus whined, his climax having made his heat a hell of a lot stronger. “Sans Please _Please_ Fuck Me.” 

Sans pulled back to pull his cock out hissing when it was finally freed from its confines. He tried to hold his brother still enough to line himself up, letting out a low moan when their magic touched for the first time. It reminded him that despite the heat, he was probably Papyrus’s first and he wanted it to be perfect. More importantly, he wanted Papyrus to know he wasn’t just doing this out of instinct. He loved his brother, so much that it used to scare him. But now they were in this together, damn the consequences. 

“i want ya so much pap, i don’t care about the heat, i will always want you.” Sans smiled up at his brother’s face, waiting for that flash of recognition before he started pushing himself inside. Papyrus’s keened and arched up into him, the legs he had hooked behind his back willing Sans to keep going deeper. “holy shit, feels incredible. it’s like ya sucking me in.” Sans babbled when he hilted inside Papyrus, listening to his whines for more and eagerly obliging. “ya doing so well. fucking take me bro, ya so fucking good.”

“Please! Fuck Me Hard- Oh My God!” Papyrus bucked up into him when Red rutted against him harder than before.“Yes! Yes! More, Saaaans!!!”

Sans growled and let go of the last shred of control he had, pounding into his brother as hard as his body would let him. He couldn’t believe they were actually doing this, he was giddy with excitement and already feeling the build-up of his own climax. Sans thought himself stupid for denying them both for so long, their stubborn natures getting in the way of experiencing something incredible together. Screw what everyone else thought, Sans loved his brother more than anything else in the world and couldn’t get over everything about him. If Papyrus’s frantic cries were anything to go by, he felt the exact same way too. 

“Sans! I-I- Lo- ah! I Love You- I Love You!”

Sans had heard his brother say he loved him before, infrequently and only when they had managed to survive something awful. But this was entirely different, and _gods_ he had never wanted his brother more. 

“me too papy, i love you so goddamn much, oh fuck-” Sans’s eyes went wide as he felt the base of his cock swelling. Clearly his brother’s heat was making his magic respond with the need to mate. But he didn’t feel half as worried as he should have been, he _wanted_ this.“ i’m gonna cum. fuck i’m gonna cum”

“You’re Getting Bigger, What?” Papyrus looked confused before his eyes rolled back in their sockets as the increase in pressure made him see stars. “Oh Gods, Ah!”

“yeah its a knot, it happens to make sure we-” Sans tried to explain calmly but he could feel Papyrus start to tighten around him, making him lose all sense of explaining things gently. “fuck pap it’s so all my cum stays inside- so i can keep filling you up- oh fuck- so i can fucking breed you are you sure-”

“Yes! I Want That-” Tears falling down Papyrus’s sockets, but there was no hint of a lie in his face. “Please, Sans!”

Sans bared his teeth and roared like a beast, pushing his knot into his brother’s willing pussy as he fell over the edge. “shit-fuck-shit -cumming! fuck!” 

Sans wasn’t sure who screamed louder when he fell forward onto his brother, both of their climaxes making them shake all over. He felt his cum squirting into his brother’s cunt over and over as his inner walls milked him dry. Since he couldn’t pull out Sans carefully moved his brother into his lap so that they could kiss again. It was more tender than before, but their passion for each other hadn’t been dampened at all. It was quite obvious that tonight was far from over, and even though Sans was exhausted he couldn’t wait. 

“So We’re Stuck Like This?” Papyrus asked as he tried to adjust himself in his brother’s lap while his oversensitive magic was still filled to the brim. “It-It’s Hardly Sanitary.” 

“naa boss, what we did was real dirty.” Sans said with a wink, making his brother roll his eyes. He had expected at least some awkwardness while they got used to going from brothers to lovers, but it was hard to comprehend when everything about Papyrus made him so happy. “ let's just enjoy it yeah? haven’t held ya like this in years.”

Papyrus sighed heavily, wrapping his arms around Sans hard enough to knock the air out of him. “I Don’t Want To Go Back To That. I Want More Of This.” he mumbled into Sans, bringing their bodies as close as possible. “I Really Do Love You Brother.”

Sans felt tears in his sockets again, out of relief more than everything. They may have been through hell the last few days, but it was totally worth it to experience this. He didn’t need to choose his words for his answer either, Sans knew exactly what he wanted to say.

“love ya too, papyrus.”

**Author's Note:**

> extra warning, there is reference to off-screen assault that could be seen as sexual. it isn't successful, and it's a very small part when Papyrus gets home and it is soon forgotten. It is because of the heat and monsters in this story being unable to control themselves coupled with the environment of Underfell so while it may be brushed off here irl would be more serious- please take care in reading if this content would upset you.  
> that being said i hope you enjoyed the story!


End file.
